1. Field of the Invention
The present invention is related to regeneration of spent catalysts used in hydrofining operations, and, more particularly, to the regeneration of spent catalysts employed in hydrodesulfurization and hydrodenitrogenation by treatment with a gaseous sulfur-containing reagent followed by extraction with a heteropoly acid.
2. Description of the Prior Art
As is well-known, hydrodesulfurization of, e.g., petroleum fractions, is carried out employing an alumina catalyst support impregnated with Group VIB and VIII metals such as molybdenum and cobalt. While such catalysts are extremely useful in reducing sulfur content, the effectiveness of these catalysts is diminished during the course of processing as a result of two problems. First, carbon residues (coke) deposit on the surface of the catalyst, thereby reducing the effective surface area and pore size for catalytic activity. Second, heavy metals in the feedstock, such as nickel and, more importantly, vanadium, tend to poison the catalyst, again resulting in reduced catalytic activity. During the course of processing, the temperature is generally increased at a predetermined rate in order to compensate for the coking and poisoning effects. However, as the temperature is increased, a greater percentage of the feedstock is cracked, thereby resulting in a decrease of desirable product. Further, the increase in processing temperature results in an increase in the deposition rate of carbon residues.
Regeneration of spent catalysts has been the subject of much investigation, and, consequently, a number of approaches for regenerating hydrodesulfurization catalysts have been developed. In particular, acidic media, such as mineral acids, anhydrous HF, oxalic acid and other carboxylic acids and acidified amine solutions, and oxidizing media, such as hydrogen peroxide, have been employed. However, such methods either (1) result in removal of unacceptable amounts of catalytically active metals, e.g., cobalt and molybdenum, as well as some solubilization of the support material, e.g., alumina, or (2) do not regenerate a sufficiently high degree of catalytic activity, so the catalytic activity of the regenerated catalyst is considerably inferior to that of fresh catalyst.
A sulfurous atmosphere at a pressure of at least 0.10 atmospheres and at a temperature of between 400.degree. and 825.degree. C. has also been employed to regenerate the spent metal catalyst; see U.S. Pat. No. b 4,014,815, issued Mar. 29, 1977. The process is employed for a time sufficient for the selective formation of sulfides of carbon and vanadium, which sulfides are characterized by having a high volatility and solubility in the sulfurous atmosphere and are selectively removed from the catalyst at the temperature of the process. While the disclosed process is useful in regeneration of spent catalysts, it requires highly corrosive conditions and formation and very specific volatile sulfide species. The temperatures required for vanadium removal at acceptable rates may damage the catalyst support.